1. Field of the Invention
The present invention relates to an optical position measuring instrument.
2. Discussion of Related Art
In conventional optical position measuring instruments, which for generating phase-shifted scanning signals use what is known as polarization coding, the two interfering partial beams are polarized perpendicularly to one another. To that end, typically separate polarization-optical components in the scanning unit are disposed in the beam path of the partial beams. The polarization directions of the two partial beams are each fixedly specified, by way of the polarization-optical components disposed in the respective beam paths.
In FIGS. 1a and 1b, the scanning beam path is shown in various views in a first variant of known optical position measuring instruments, which generate phase-shifted scanning signals by means of polarization coding. FIG. 1a shows the scanning beam path as far as the scale M, and FIG. 1b shows the scanning beam path from the scale M onward.
A light source L, for instance a suitable laser light source, emits a beam, linearly polarized at 45° to the X and Y axes, to a scanning plate A with a scanning grating AG. The partial beams, split into the +1st and −1st order of magnitude, propagate in the direction of the scale M, are diffracted by the scale grating MG and reflected back, and polarized perpendicularly to one another by the polarizers P5, P6. After the superposition of the two partial beams by the scanning grating AG, the resultant beam reaches a detection unit D. It includes a polarization-neutral beam splitter BS, a λ/4 retardation plate WP, the polarizing beam splitters PBS1 and PBS2, as well as the linear polarizers P1, . . . , P4 and the associated optoelectronic detector elements PE1-PE4. Because of the polarization-optical components provided in the partial beams, the optoelectronic detector elements PE1, . . . , PE4 detect different polarization states of the resultant beam striking the detection unit D. One such detection unit for optical position measuring instruments is known for instance from FIG. 7 of German Patent Disclosure DE 2127483 A.
For further representation of polarization states in optical position measuring instruments, the so-called Poincaré representation will be used hereinafter, as is shown in FIG. 2. Arbitrary polarization states are represented as a point on the surface of a sphere in an abstract coordinate system X′Y′Z′. The abstract coordinate system X′Y′Z′ here has nothing to do with the spatial coordinate system XYZ of the respective position measuring instrument; all the linear polarization states are located in the equatorial plane X′Y′. Along the equator, the polarization axis rotates by 180°. The points PX+ and PX− represent a horizontal and vertical linear polarization state, respectively, and the points PY+ and PY− represent a linear polarization state inclined by +45° and −45°, respectively. The poles PZ+ and PZ− are assigned a left- and right-circular polarization state, respectively. All the orthogonal polarization states are always located at diametrically opposed points. More-detailed information on the Poincaré representation can be found for instance in M. Born, E. Wolf: Principles of Optics, pp. 32, 33, Cambridge University Press, 1999.
In FIG. 2, the polarization states of the known position measuring instrument are shown in FIGS. 1a, 1b on the Poincaré sphere. The polarization states of the two linearly polarized partial beams are represented in this drawing by reference numerals Π−1 (−1st order of magnitude of the scale M) and Π+1 (+1st order of magnitude of the scale M). The superposition of the partial beams with these polarization states results in a polarization state Π0, which moves along the great circle G in accordance with the relative phase relationship of the partial beams. The plane of the great circle G is perpendicular to the connecting line of the generating polarization states Π−1 and Π+1. The polarization states Π1-Π4 detected by the optoelectronic detector elements PE1-PE4 are located on the great circle G, in order to obtain a maximum degree of modulation of the scanning signals. They detect the following polarization states:
PE1: Π1=PY−
PE2: Π2=PY+
PE3: Π3=PZ−
PE4: Π4=PZ+
Maximum signal levels of the resultant scanning signals are always indicated by an optoelectronic detector elements PEn (n=1, . . . , 4) whenever the resultant polarization state Π0 coincides with the detector polarization state Πn of the optoelectronic detector element. The signal levels of the scanning signals are correspondingly minimal when the resultant polarization state Π0 is located diametrically opposite the detected polarization state Πn.
A second variant of known optical position measuring instruments, which generates polarization-coded phase-shifted scanning signals, is shown in FIGS. 3a and 3b. Once again, FIG. 3a shows the scanning beam path as far as the scale M, and FIG. 3b shows the scanning beam path from the scale M on. The two partial beams split by the scanning grating AG are now left- and right-circularly polarized with the aid of λ/4 retardation plates WP1 and WP2 and thus polarized again orthogonally to one another. A suitable detection unit for this variant of optical position measuring instruments is known for instance from FIG. 10 of German Patent Disclosure DE 2127483A.
The associated polarization states are again entered into the Poincaré sphere in FIG. 4 as points Π−1 and Π+1. The polarization state Π0 resulting from the superposition, here as well, moves along a great circle G as a function of the relative phase relationship. The great circle G is now located in the X′Y′ plane, which however is perpendicular to the connecting line of the generating polarization states Π−1 and Π+1. The detection unit D in this variant contains a X12 retardation plate WP, which rotates the incident linear polarization Π0 by only 45°. Otherwise, the detection unit D is identical to the one in the first variant. The polarization states Π1-Π4 detected by the optoelectronic detector elements PE1-PE4 are:
PE1: Π1=PY−
PE2: Π2=PY+
PE3: Π3=PX−
PE4: Π4=PX+
From U.S. Pat. No. 6,914,234, an optical position measuring instrument is also known, whose scale grating has periodically modulated polarization properties. Within each graduation period of the scale grating, the incident beam is locally linearly polarized, and the polarization direction rotates over the grating period by 180′; accordingly, the polarization period is equivalent to the graduation period of the scale grating. The extent of the scanning beam is selected to be so small that only part of one graduation period of the scale grating is illuminated, so that the exiting beam has a linear polarization whose direction rotates upon a displacement of the scale. The variation of the polarization state of the exiting beam is thus equivalent to the situation shown in FIG. 4. The scanning optics described in U.S. Pat. No. 6,914,234 is thus equivalent to the scanning optics already explained from FIGS. 3a and 3b. 
In summary, it can accordingly be stated that in the polarization-coded optical position measuring instruments of the prior art, there are fixed polarization states of the superimposed partial beams, which in the Poincaré representation leads to a stationary great circle G. Phase-shifted scanning signals (3×120°, 4×90°) are generated by optoelectronic detector elements by the detection of polarization states, all of which are located in the plane of the great circle G.
For certain novel scanning optics of optical position measuring instruments, of the kind proposed for instance in German Patent Application DE 102010063216.3 of the present Applicant, there are accordingly limitations. These scanning optics are distinguished in that after a defined displacement or rotation of a component of the optical position measuring instrument, such as the scale or scanning unit, etc., the second partial beam strikes the same point on each optical component of the optical position measuring instrument that the first partial beam struck previously. Such scanning optics of optical position measuring instruments will hereinafter also be called “scanning optics without partial beam association”. In such scanning optics without partial beam association, it is not possible, at a defined point in the beam path of the optical position measuring instrument, to dispose polarization-optical components in such a way that it is always only the first or the second partial beam that is affected.